The rotating blade for a steam turbine is made from a 12Cr--Mo--Ni--V--N steel at the present time. In recent years, there is a desire to improve the thermal efficiency of the gas turbine from the viewpoint of energy saving and to make the equipment of the gas turbine more compact from the viewpoint of space savings.
To improve the thermal efficiency of a gas turbine and to make the equipment thereof more compact, it is effective to make the blades of the steam turbine longer, and for this purpose, there has been a tendency to make the length of the final stage blades of the low pressure steam turbine becomes longer every year. With such a tendency, the service condition for the blades of a steam turbine becomes strict, and as a result the 12 Cr--Mo--Ni--V--N steel is no longer sufficient in strength under the above service conditions, and therefore, it is expected that a new material will be developed having a higher strength. The strength of the material for the blades of the steam turbine is determined by its tensile strength which is a basic mechanical characteristic.
The material for the blades of a steam turbine is also required to exhibit a high toughness in addition to a high strength for ensuring safety against breakage.
As a structural material having a tensile strength higher than that of the conventional 12 Cr--Mo--Ni--V--N steel (martensite based steel), there are generally known a Ni based alloy and a Co based alloy; however, such materials are undesirable as blade materials because of their poor working ability at hot temperatures, poor machinability, and periodic damping characteristic.
A disk material for a gas turbine is known, for example, from Japanese Patent Laid-open Nos. Sho 63-171856 and Hei 4-120246.
In the conventional steam turbine, the maximum steam temperature has been set at 566.degree. C. and the maximum steam pressure has been set at 246 atg.
However, from the viewpoint of exhaustion of fossil fuel such as mineral oil or coal, energy saving, and prevention of environmental pollution, it is desired to increase the efficiency of the thermal power-generation plant, and to increase the efficiency of power-generation, it is most effective to increase the steam temperature of the steam turbine. A suitable material for such a high efficient ultra-high temperature steam turbine is known from Japanese Patent Laid-open No. Hei 7-233704.
The present invention has been made to cope with the recent trend to make the blades of a low pressure steam turbine longer. A suitable material for the rotating blades for a steam turbine is not disclosed in Japanese Patent Laid-open Nos. Sho 63-171856 and Hei 4-120246 at all.
Japanese Patent Laid-open No. Hei 7-233704 discloses a rotor material, a casing material, and the like; however, as described above, the document does not describe a 12% Cr based martensite steel for a final stage rotating blade for a high pressure side turbine-intermediate pressure side turbine integral type steam turbine and a low pressure steam turbine which are operated at high temperatures.
An object of the present invention is to provide a steam turbine operable at a high temperature in a range of 600 to 660.degree. C. by use of ferrite based heat resisting steels, to thereby enhance the thermal efficiency, and a steam turbine power-generation plant using the steam turbine.
Another object of the present invention is to provide a steam turbine operable at each operating temperature in a range of 600 to 660.degree. C. with its basic structure being substantially not changed, and a steam turbine power-generation plant using the steam turbine.